Liquid crystal panels (e.g. dot inversion panels) may use a single-inductor dual-output power. A plurality of boost voltage converters sharing a single inductor may produce single-inductor dual-output power, which may be used to apply positive and negative power to a liquid crystal panel.
Example FIG. 1 illustrates a circuit diagram of a single-inductor dual-output power circuit, in accordance with the related art. The single-inductor dual-output power circuit 100 may include a boost voltage converter 110 and a buck-boost voltage converter 120. The boost voltage converter 110 may apply a positive voltage DDVDH for a liquid-crystal drive to a load R1. A buck-boost voltage converter 120 may apply a negative voltage DDVDL to a load R2. The boost voltage converter 110 and the buck-boost voltage converter 120 share a first switch element M1, a second switch element M2, and an inductor L. The boost voltage converter 110 and the buck-boost voltage converter 120 include diodes D1 and D2 for performing rectification functions, capacitors C1 and C2 for performing smoothing functions, serial buffers B1 through B4 for adjusting rising and falling times, respectively.
A single-inductor dual-output power circuit 100 may allow the first switch element M1 and the second switch element M1 to perform alternate switching operations by a first switch control signal CNT1 and a second switch control signal CNT2, which may be applied from an apparatus for controlling a pulse frequency modulation. The result of the signal CNT1 may be generation of the positive voltage DDVDH and the negative voltage DDVDL. The generated positive and negative voltages DDVDH and DDVDL may be provided to the loads R1 and R2, respectively.
If both the first switch element M1 and the second switch element M2 are turned-on, a current flows through the inductor L. A period when both the first switch element M1 and the second switch element M2 are turned-on may be referred to as a maximum on-time. An on-time pulse signal used for the maximum on-time may be set to have a preset period.
If the second switch element M1 is turned-on and the first switch element M1 is turned-off, the current passing through the inductor L is charged into the first capacitor C1, which may cause a rise of the positive voltage DDVDH. If the first switch element M1 is turned-on and the second switch element M2 is turned-off, the current passing through the inductor L is charged into the second capacitor C2, which causes a drop in the negative voltage DDVDL. When both the first and second switch elements M1 and M2 are turned-off, the current passing through the inductor L is charged into both the first and second capacitors C1 and C2, which may cause the positive voltage DDVDH to rise and the negative voltage DDVDL to fall. Accordingly, a period when at least one of the first and second switch elements M1 and M2 is turned-off may be referred to as the minimum off-time. An off-time pulse signal may be used for defining the minimum off-time to a preset period.
However, when the on-time and off-time pulse signals are fixed in period, a peak current flowing through the inductor L increases in order to meet a desired current capacity of the converter. As a result, noise and electromagnetic interference (EMI) may be generated in the converted voltages.
FIG. 2 illustrates a waveform diagram of a current of the inductor when a maximum on-time pulse signal and a minimum off-time pulse signal have fixed periods, in accordance with the related art. A first interval INTERVAL1 indicates a period that the first switch element M1 is alternately turned on and turned off while the second switch element M2 maintains a turned-on state. A second interval INTERVAL2 indicates a period that both the first and second switch elements M1 and M2 are alternately turned on and turned off.
When the positive voltage DDVDH rises in the first interval INTERVAL1, the current IL of the inductor L may decreases with an inclination of (DDVDH−VCI)/L even though the turning-on voltage of the first diode D1 is disregarded. In the second interval, INTERVAL2 when the positive voltage DDVDH rises and the negative voltage DDVDL falls, the current IL of the inductor L decreases with an inclination of ‘(DDVDH+|DDVDL|)/L’ even though the turning-on voltages of the first and second diodes D1 and D2 are disregarded. Accordingly, it may be necessary to flow a considerable quantity of peak current through the inductor L, in order to meet a desired current capacity of the converter. In addition, since the maximum on-time and minimum off-time pulse signals are fixed in period, the inductor current IL may suddenly increase depending on the controlled states, as shown in FIG. 2.